1 Introduction Wire EDM machines have been widely used in mold processing. Since the WEDM process is generally performed after heat treatment, the disadvantages of heat treatment deformation and surface decarburization are avoided. There are many mold manufacturing units that are often wire cut and assembled with a little grinding. Some of them are assembled directly without grinding. As a result, chipping, breaking, and chipping often occur. Even if the above phenomenon does not occur, the sharpening life of the die is not long. This paper discusses the stress state of the surface of the module after the wire cutting and the method of eliminating the stress and improving the surface quality.
2 wire cutting after the stress state of the surface of the part, (China gold packet network is the iron and steel canning industry portal) many molds are used fast wire cutting machine, wire cutting surface roughness Ra ≥ 2. 5μm, hardness distribution And the state of internal stress is very poor.
When the wire cutting machine processes, the current density in the discharge area is as high as 10000A/mm2, the temperature is as high as 10000 to 12000°C, and the filled medium liquid is rapidly cooled, so that the surface hardness of the cut surface is only about 20HRC, and the hardness of the internal quench layer is as high as 70HRC or more. After the heat affected zone, it is the original hardness area. It is particularly serious that the inside of the raw material is in tensile stress due to quenching, and the thermal stress generated by wire cutting is also tensile stress. The superposition of the two stresses can easily reach the limit of the material's strength to produce micro-cracks, which greatly shortens the life of the die. Cutting cannot be used as the final processing step for punch and die. As can be seen from Figure 1, the hardness profile of the cut section of the CrWMn material after wire cutting and the change in hardness after tempering and aging.
3 Elimination of stress caused by wire cutting
3.1 Grinding and Removing White Layers At present, most of the mold processing units are cut in-line, and the gray layer (ie, white layer) of the surface 20HRC is removed by grinding and then assembled. Although this method can remove the white layer with low hardness, it does not change the stress state in the stress region caused by wire cutting. Even if the grinding margin after wire cutting is increased, the high hardness of the hard layer (up to 70HRC) makes grinding difficult. Excessive amount of grinding can easily damage the part geometry. The high hardness layer produced by wire cutting does not improve the die life because the brittleness is the fundamental cause of cracks and chipping.
3.2 Tempering After online cutting, the white layer on the surface of the part is ground and then tempered at 160-180°C for 2 hours. The high hard layer below the white layer can be reduced by 5-6HRC, and the thermal stress caused by wire cutting is also reduced. , (China Gold Packaging Network is the printing iron cans industry portal site) to improve the toughness of the die, extending the service life. However, due to the short tempering time and incomplete elimination of thermal stress, die life is not very satisfactory.
3.3 Grinding After grinding, the white layer and hard layer can be removed and the life of the die can be improved. Because the thermal stress generated during grinding is also tensile stress, superimposed with the thermal stress generated by wire cutting, it will undoubtedly aggravate die damage. If after grinding, low-temperature aging treatment, the effect of stress can be eliminated, the die toughness can be significantly improved, and die life can be improved. Because most of the punches with complicated geometric shapes are processed by wire cutting, the grinding of complex-shaped punches must use expensive coordinate grinding machines and optical curve grinding machines. These two types of equipment are generally not available to manufacturers, so promotion is difficult.
3.4 After shot peening, low temperature tempering shot peening can transform the retained austenite of the wire cutting incision into martensite, improve the strength and hardness of the die, change the stress state of the surface layer, reduce the tensile stress, and even become The state of compressive stress makes crack initiation and expansion difficult, combined with low temperature tempering, eliminating the tensile stress in the quenched layer, can increase the life of the die by 10 to 20 times. Shot peening is limited by the condition of the equipment and the shape of the die part (inner surface) and is difficult to use universally.
3.5 Grinding and then low-temperature aging treatment After the surface of the wire is ground, the high-hard layer has been basically removed, and then 120-150°C×5～10h low-temperature aging treatment (also called low-temperature tempering treatment) can also be performed at 160-180°C. ×4～6h Low temperature tempering treatment. This eliminates the internal tensile stress of the quenched layer, while the hardness decreases slightly (the latter decreases slightly in hardness), but it greatly improves the toughness, reduces the brittleness, and increases the die life by more than 2 times. This method is simple, effective, very obvious, easy to promote. After the cold die and the cold die have pressed several parts, the internal stress of the die has accumulated very high. This method can also use this method to eliminate the stress, improve the toughness, and increase the service life.
4 Example of application The molybdenum used the CrWMn die. After the wire cutting, the following tests were performed. The service life is very different.
(1) Directly used for punching, sharpening life of 10742 times.
(2) After tempering at 160 °C for 2 hours, the life of sharpening is 11180 times.
(3) Grind white layer, sharpening life only 4860 times.
(4) Grind the white layer and temper it at 160 °C for 2 hours. The sharpening life is 7450 times.
(5) Grinding, grinding life of 28743 times.
(6) Tempering after 160°C×2h after shot blasting, the sharpening life reaches 220000 times. The Cr12 material is used to process the die (as shown in Fig. 2), and the Kovar is 镦b = 800MPa. The original process is 960 ~ 980 °C oil quenching + 160 ~ 180 °C × 2h tempering, wire cutting After grinding by the locksmith, direct compression, the service life of only a few dozen break.
If the warp is cut and then ground, it is tempered at 160-180°C for 2 hours. The service life is several hundred and it is still broken. After grinding by wire cutting, aging treatment is performed at 120°C to 150°C for 6 to 10 hours. The service life is more than 3,000 pieces and it is still in use.
5 Regarding the grinding problem It can be seen from Fig. 3 that the wear of the male and female molds occurs simultaneously in both directions along the side (the wire cut surface) and the end surface (the sharpened surface), and the former wears slightly more than the latter. The lower the surface roughness values ​​of the convex and concave molds, the higher the fatigue resistance, and each time the roughness value is reduced by 1 level, the life can be increased by a factor of two. Therefore, when both convex and concave molds are polished, both must be considered.
Some people have done such an experiment and used two convex and concave molds with the same structure to punch the same material. One group only grinds convex and concave mold sides and grinds end surfaces, while the other group grinds side surfaces and grinds after grinding. Experimental results show that the latter's life is nearly 1 times higher than the former. The above experimental results are not difficult to understand, because when the blank is punched, the material flows toward the edge of the convex and concave die as the punch enters the depth of the blank, until the punch edge and the die edge produce When cracks overlap. When the material flows, the convex and concave die end faces generate large frictional forces, and the magnitude of the frictional force depends to a large extent on the roughness of the convex and concave die end faces. (China Golden Packing Network is a portal site of the printed iron can making industry) Therefore, polishing the convex and concave die end surfaces is beneficial to improving the die life, especially the small and medium-sized die with complex shapes and high precision requirements.
6 Conclusion Convex, concave die after the online processing method, the die life has a great impact. How to eliminate the brittleness produced by wire cutting and improve the toughness, the best method is shot blasting + low temperature tempering, followed by grinding + grinding + low temperature tempering, again grinding + low temperature aging treatment, the various units according to their own specific Situation selection.
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